When a molecular ion generated from a sample molecule is made to move in a gas medium (or liquid medium) under the effect of an electric field, the ion moves at a speed proportional to its mobility which is determined by the intensity of the electric field, size of the molecule and other factors. Ion mobility spectrometry (IMS) is a measurement method in which this mobility is utilized for an analysis of sample molecules. FIG. 3 is a schematic configuration diagram of a conventional and common type of ion mobility spectrometer (for example, see Patent Literature 1).
This ion mobility spectrometer includes: an ion source 1 for ionizing component molecules in a sample; a drift region 4 which is provided, for example, within a cylindrical housing (not shown), for measuring the ion mobility; and a detector 5 for detecting ions which have traveled through the drift region 4. Additionally, in order to send the ions generated by the ion source 1 into the drift region 4 in a pulsed form with a short duration, a shutter gate 3 is provided at the entrance to the drift region 4. The atmosphere inside of the housing is maintained at atmospheric pressure or low vacuum of approximately 100 Pa. A uniform electric field having a downward potential gradient (for accelerating ions) in the moving direction of the ions (in FIG. 3, the Z-direction) is formed within the drift region 4 by DC voltages respectively applied to a number of ring-shaped electrodes 2a included in a drift-electrode group 2 arranged within the drift region 4. A flow of neutral diffusion gas is formed in the opposite direction to the direction of the acceleration by the electric field.
The ions generated in the ion source 1 are temporarily dammed by the shutter gate 3. The shutter gate 3 is subsequently opened for a short period of time, whereupon the ions in a packet-like form are introduced into the drift region 4. Colliding with the counterflowing diffusion gas within the drift region 4, the introduced ions are driven forward by the accelerating electric field. Those ions are temporally separated according to their ion mobilities, which depend on the size, steric structure, electric charge and other properties of the individual ions. Accordingly, ions with different ion mobilities reach the detector 5 having certain intervals of time. If the electric field within the drift region 4 is uniform, the collision cross-section between an ion and the diffusion gas can be estimated from the drift time required for the ion to pass through the drift region 4.
As the shutter gate 3 for controlling the blockage and passage of the ions, a gate electrode described in Patent Literature 2, Non Patent Literature or other documents is often used, which is generally called the “Bradbury-Nielsen gate”. FIG. 4 is a schematic perspective view of a shutter gate employing the Bradbury-Nielsen gate described in Patent Literature 2.
In the shown example, two comb electrodes 231 and 232 created by winding, etching or other appropriate techniques are adhered to one surface of a plate-shaped ceramic base 21 in which a circular opening 22 is formed. One comb electrode 231 is connected to a positive-voltage input terminal 241, and the other comb electrode 232 is connected to a negative-voltage input terminal 242. The resulting electrode system has thin-wire electrode lines stretched over the opening 22, with positive and negative voltages alternately applied to the individual electrode lines aligned in the x-direction. The voltages applied to the two comb electrodes 231 and 232 through the positive and negative voltage input terminals 241 and 242 are controlled to create an ion-damming electric field within an area near the opening 22 or dissolve this electric field to allow ions to freely pass through.
In such an ion mobility spectrometer, if water vapor, solvent droplets or similar particles which have not been fully vaporized are present within the drift region 4, the ions to be subjected to the measurement come in contact with those fine particles. This constitutes a significant factor of the fluctuation of the drift time. Therefore, in order to remove as much as possible such vapor and fine solvent droplets from the drift region 4, the drift tube which forms the drift region 4 is heated to a high temperature during the analysis. In normal cases, the heating temperature is within a range of 120-130° C. Actually, it is more preferable to increase the temperature to higher levels (150-160° C.).
However, exposing the shutter gate having the previously described structure to a high temperature causes the comb electrodes 231 and 232 to be taut or slack due to the difference in the coefficient of thermal expansion between the ceramic base 21 and the metallic comb electrodes 231 and 232. Consequently, for example, the neighboring wire electrodes stretched over the opening 22 may come in contact with each other, making the shutter unable to fully perform its function. Furthermore, the wire electrodes may be deformed or severed, in which case the shutter becomes unusable. Due to such restrictions, it has been necessary to set the temperature of the device at 120-130° C. or lower levels. A measurement with the drift tube heated to a high temperature as mentioned earlier has been impractical.